1. Field of the Invention
The present invention relates to catalyst components for olefin polymerization which can produce polyolefins, and in particular polypropylene having a low melting point, high amount of atactic content that is soluble in decalin solvent, and consequently, suitable manufacturing properties.
2. Description of the Prior Art
The homopolymers and copolymers of propylene generally have certain properties that are unsatisfactory for specific applications. It therefore becomes necessary to modify certain characteristics during the manufacture of the polypropylene to render the polymer more useful for a certain end results. For example, if the rigidity of the polymer or copolymer of propylene is improved, it is possible to reduce the thickness of the resulting molded product formed from it.
There are numerous polymerization and copolymerization processes and catalyst systems in the prior art from which it is possible to tailor a processing catalyst system to obtain a specific set of properties of a resulting polymer or copolymer. For example, in certain applications, a product with higher melt flow rate is desirable. Such a product has a lower melt viscosity than a product with a lower melt flow rate. Many polymer or copolymer fabrication processes which operate with high shear rates, such as injection molding, oriented film and thermobinded fibers, would benefit from a lower viscosity product by improving through-put rates and reducing energy costs. Generally, olefin polymers obtained by using an active catalyst component of the magnesium (MgCl2) supported type have a limited melt flow rate range and mechanical properties. As indicated, however, for certain applications, polypropylene polymers which flow readily during melting have improved processing characteristics.
The discovery of more appropriate co-catalysts or electron donors to accompany supported magnesium catalyst components has been of great benefit to improving the efficiency of the catalyst system and the quality control of the polymer product. In such catalyst systems, the cocatalyst activates the catalyst and provides initiation of a polymer chain. The cocatalyst that has historically worked well with magnesium supported catalysts is organoaluminum compounds, most typically triethylaluminum (xe2x80x9cTEALxe2x80x9d), or other trialkyl aluminum compounds. Examples of other useful organoaluminum compounds include an alkylaluminum dihalide, a trialkoxyaluminum, a dialkylaluminum halide and a triisobutyl aluminum.
An electron donor compound is used in the polymerization reactor to control the stereoregularity and form of the polymer. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or groups of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor can lead to significant improvements in the properties of the polymer product such as molecular weight distribution and melt flow. Discovery of a specific group of electron donors for magnesium supported catalysts that would provide beneficial results would be highly advantageous.
Electron donors have been used to improve the melt flow rate (xe2x80x9cMFRxe2x80x9d) characteristics of polypropylene polymers by controlling the polymer chain length and position of defects along those chains. However, the drawback to adding an electron donor to the catalyst and polymerization system is that the melting point of the final polymer product often increases with addition of the electron donor. This is a disadvantage in many processing procedures and applications of the polypropylene product. Further, decalin solubles (a measure of the amount of crystallinity) are often depressed upon the addition of an electron donor, which is a disadvantage when a polymer having less crystallinity is desired.
Another disadvantage to the addition of a single, or certain multiples, of electron donors is that the melting point and decalin soluble profiles are increased. The melting point profile for a given electron donor is the profile (or plot) of the melting points of the polymer generated by the catalyst system as a function of the electron donor concentration. Likewise, the decalin solubles profile is a plot of the decalin solubles (amount of solubles) as a function of the electron donor concentration. When an electron donor, or certain combinations of electron donors are added to the catalyst system, the profile is increased such that the addition of a small amount of electron donor has a large effect upon the melting point or decalin soluble content. This is a disadvantage in commercial processes, where the exact amount of added electron donor is often hard to control. An ideal catalyst system would be one where the added amount of electron donor, or electron donor mixture, would have a relatively small effect on the melting point or decalin solubles, A system having a lower melting point and decalin solubles profile is thus ideal.
The present invention is directed towards the surprising finding that the use of at least two different organosilicon compounds as electron donors in combination with a magnesium supported catalyst is capable of generating polypropylene polymers having a moderately broad molecular weight distribution and desirable MFRs, while lowering the overall melting point of the final product. Further, it has been surprisingly found that the melting point profile and decalin solubles profile are advantageously lowered upon the use of at least two different organosilicon compounds.
Embodiments of the present invention include a catalyst system for the polymerization of olefins comprising:
(a) a solid magnesium supported titanium catalyst component;
(b) an organoaluminum co-catalyst; and
(c) a mixture of at least two electron donors, wherein the mixture comprises methylcyclohexyldimethoxy silane and at least one secondary electron donor, or
(d) a mixture of at least three electron donors.
The invention also includes a process for producing polyolefins using the catalyst system. The secondary electron donor or the at least three electron donors are selected from the group consisting of tetraethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, dimethyldimethoxysilane, methyltrimethoxysilane, and dicyclopentyldimethoxysilane, and a mixture thereof. In one embodiment of the catalyst, the solid catalyst is a magnesium supported TiCl4 catalyst comprising TiCl4 and the organoaluminum co-catalyst triethylaluminum.
In order to achieve the high decalin solubles and low melting points of olefins polymerized by the method of the invention, the relative mole ratios of the electron donors is adjusted. In one embodiment, propylene monomers are polymerized using the method of the invention, and the resultant polypropylene has a melting point in the range from 160xc2x0 C. to 164xc2x0 C. In another embodiment, the decalin solubles value of the resultant polypropylene is 50% to 100% greater than the value when one electron donor is present. Due to the improved characteristics of the polyolefins produced by the method of the invention, improved articles such as films and injection molded articles can be made.